JP5778702B2 - Manufacture of synthetic silica glass - Google Patents

Description

  The present invention relates to the production of synthetic silica glass, which may be pure or doped. In particular, the present invention provides a liquid silica precursor material within the flame of a synthetic burner to provide silica particles (eg, nanoparticles) that may be mixed with one or more dopants such as metal oxides. Relates to the production of such glasses by controlled combustion of sprays. The particles can be deposited to give a porous soot body for subsequent consolidation to glass without pores, or directly to give glass without pores more It can be deposited at high temperatures.

  Furthermore, but not limited to, the present invention relates to the production of high quality synthetic vitreous silica, substantially free of microbubbles or inclusions, and uniformly with one or more metal oxides. Being doped, the oxide can be refractory and can be difficult to dissolve in silica glass. The method can additionally be used to provide silica glasses doped with non-metallic dopants, including, for example, oxides of boron and phosphorus.

  The glass product produced according to the invention can be used, for example, as an optical material. In such cases, any dopant will provide useful properties. For example, cerium (optionally also added with titanium) can be used to introduce controlled light absorption at specific wavelengths: or rare earth metals such as europium, neodymium, samarium, ytterbium are desirable photo It can be used to introduce luminescence, potential laser activity, etc.

  Such glasses can be used in the manufacture of optical fibers or waveguides, both planar, etc., and can be used on a large scale due to their optical properties. Doping with oxides of certain rare earth metals, such as yttrium, is resistant to etching with fluorine-containing gases in a plasma etching process during semiconductor manufacturing, with or without a second metal such as aluminum as a co-dopant. Can be granted. Doping with titanium is used to produce glass with a low (or even zero) coefficient of thermal expansion, optionally in the presence of other metal species. Doping with certain species can be used to increase the viscosity of glass and the like. Although this list is not considered broad, there are potential applications in the optics, semiconductor and other industries for the glass produced by the method of the present invention.

  In recent years, it has become routine to produce synthetic silica glass by vapor deposition, where a vapor of a silicon compound is fed to a burner, optionally mixed with one or more dopant precursors, It is then converted to oxide nanoparticles in the flame. These particles are then deposited as porous bodies (known as soot bodies) that can be fired into pore-free glass, or to form glass directly at relatively high temperatures (so-called direct quartz method). The

  The demand for volatile precursors is widespread with silicon tetrachloride as the majority of silica precursors, and various metal and non-metal chlorides as precursors for dopants when product doping is required. It is used across.

  However, using chlorine-free volatile silica precursors instead of silicon tetrachloride has several advantages, especially the absence of acidic by-products in the effluent gas. The potential use of alkoxides was recognized in earlier work on synthetic silica (eg US 2,272,342), but more recently siloxanes have a very high silicon content and are flammable, chlorine-free precursors. Has been used as. In particular, hexamethyldisiloxane (HMDS), octamethylcyclotetrasiloxane (OMCTS, also known as D4) and decamethylcyclotetrasiloxane (DMCPS, or D5) (eg EP0,463,045 and US5,043,002 (US RE39535)) ).

  Although the precursor vapor supply is considered conventional, there are many examples where a synthetic flame is generated, and instead of supplying the precursor as a vapor, one or more precursors are sprayed into droplets. It is supplied as a spray, where it evaporates in a flame and is then converted to silica nanoparticles by oxidation or hydrolysis.

Thus, the production of pure and doped silica by deposition of porous deposits from sprays of precursors into a flame was proposed in JP54-142317, JP55-95638 and JP56-14438. Various oxyhydrogen burner shapes were proposed in JP55-95638. In most of these, the liquid, which can be a mixture of silica precursor and dopant precursor, is atomized into an oxyhydrogen flame by a gas blast (air or venturi) atomizer and the product is porous soot that is fired into glass. It was collected as a body. The silica precursor contained tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS), and the dopant precursor contained PO (OC ₂ H ₅ ) ₃ and B (OC ₂ H ₅ ) ₃ . In addition, it has been proposed that the metal dopant can be introduced as a separate spray of an aqueous or alcohol solution and used to adjust the refractive index of the product glass to produce an optical fiber preform. Other anticipated applications included glass ceramics and laser active glass compositions.

  The ability to supply the dopant species as an aqueous solution rather than as an expensive or labile organometallic species allows for significant cost savings, but the glass quality resulting from supplying two separate sprays to the flame. There must be doubt.

  The spraying method is further developed in JP56-14438, if the liquid precursor pneumatic spray is replaced by the use of an ultrasonic spray horn, or if a combination of pneumatic and ultrasonic spray is used. It has been mentioned that increased deposition rates are possible. A soot deposition rate of 400 g / hr was reported, but the effectiveness of converting a spray containing separate droplets of silica precursor and dopant was questionable and no indication was made about the quality of the glass produced by this method. .

Spray combustion to generate pure and doped silica glass was continued by other researchers. JP60-96591 can supply the spray generated by the insufflation of tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) to the center of a co-annular oxyhydrogen flame and fire it into glass. It is described to obtain a porous deposit of pure silica. More recently, US 5,110,335 proposed ultrasonic spraying using SiCl ₄ or TEOS as the precursor liquid, as well as combustion in acid methane flames. Spray combustion of chlorine-free siloxane precursors has also been described in many patents for both pure and doped synthetic silica glasses (eg, EP 0,868,401, US 6,260,385, US 6,588,230, US 6,374,642). , US 6,739,156 and US 6,837,076).

  Despite many patents in the literature describing the deposition of synthetic silica from flames fed with liquid precursor sprays, most synthetic vitreous silica produced to date is supplied in vapor form. It is noteworthy that it is still produced by combustion of the precursor. Nevertheless, this is an advantage that appears to arise from the movement of the liquid precursor to the spray supply. All that appears to be required for spray combustion is the installation of a liquid storage and handling system, a metering pump, and the necessary gas supply with a suitable flow controller, as well as a suitable burner or burner arrangement. For the steam supply, an appropriate heated evaporator for the silica and dopant precursors and an appropriate heated line for the steam supply to the burner must be additionally provided, all at the appropriate temperature It is necessary to provide monitoring and adjustment facilities. However, the production of high quality glass by spray combustion is not without problems.

  In US 6,260,385 (column 2, line 61 to column 3, line 8), a simple atomization burner based on air blast atomization is wart-shaped if the atomization gas is not supplied at high speed. -like) It is described that an insufficient soot body can be provided with growth. This facilitates the spraying of relatively small droplets and reduces the risk of insufficient evaporation and combustion before the droplets impact the substrate, but increasing flame turbulence has the disadvantage of reducing soot deposition efficiency, Can cause other problems. In this patent it was described that these problems were overcome by using ultrasonic atomization; however, this choice was abandoned due to the so-called effervescent atomization burner. It is said that the gas dissolves under pressure in the precursor liquid, and the dissolved gas forms bubbles in the appearing droplets, so that it is broken down into relatively small droplets as the liquid exiting the nozzle. However, it was necessary to enclose the spray in the form of a tubular containment (referred to as a “rail”) in order to achieve sufficient confinement of the flame for efficient silica soot deposition. It looks like There was no illustration of the application of this concept.

  Further potential problems arising during siloxane spray combustion were discussed in US 6,374,642. If the precursor droplet is too large, the soot body will be defective. The droplet hits the substrate before it is fully evaporated, and then oxidation occurs on the substrate surface. Alternatively, partial evaporation and oxidation occurs in flight, but then silica nucleation can occur around the partially evaporated droplets, resulting in oversized soot particles. For these reasons, an ultrasonic atomizer is coupled to the burner as a means to achieve fine droplet size without the need for an extra flow of atomizing gas. Furthermore, the reaction plume from the synthesis burner is surrounded by a flame from two or more auxiliary burners, ensuring sufficient evaporation and combustion of the precursor before reaching the substrate. Was intended to be. These additions greatly increase the complexity of the process. Some suggestions have been made regarding the possible gas flow and the operating distance between the burner and the substrate, but the quality of the resulting gas is not clear. It is required to produce a doped synthetic silica glass, where if the dopant oxide is a refractory with a high melting point and / or boiling point, a glass that is evenly doped and is free of bubbles and contaminants. If it is necessary to produce, the method of spraying the two liquids together into the flame will further improve if the precursors are sufficiently evaporated prior to combustion or in the course of combustion and cannot be mixed uniformly. Cause problems. Otherwise, one or both of the precursors can be converted to solid or liquid particles, the size of which depends on the size of the droplets in the spray, as well as the evaporation of all relevant species. Without homogeneous nucleation in the flame and growth of the doped glass, this method usually cannot be expected to produce a doped glass with the required uniformity in atomic size. is there. These points are described in a recent patent (US 7,624,596), but the practicality of the proposed solution (ultrasonic expansion with a Laval nozzle) is unclear and there are no examples. Not shown.

  One possible reason for the apparent lack of atomization is that it is relatively difficult to achieve uniform droplet size uniform spray in a uniform gas stream, and Generate highly oxidized nanoparticles at a high concentration for collection on the substrate, which can be precursor unburned particles, or oversized particles of oxides or precursor pyrolysis products (eg silicon carbide) Or oxycarbide, which can lead to defects in the product glass), to achieve evaporation of all droplets in a well defined area of the flame and then controlled combustion as required It is. In the past, these requirements have been more easily achieved in a flame of a synthetic burner that supplies all precursor species in vapor form.

  For doped silica deposition using two or more precursors, there is an additional problem that ensures uniform mixing of the constituent species at a constant repeatable rate. If the precursors are miscible and can be utilized as chemically compatible liquids, they can be supplied as a solution to a suitable spray nozzle and the resulting spray is introduced into the soot deposition flame. Thus, the production of silica doped with tantalum oxide has been proposed by spraying hexamethyldisiloxane (HMDS) of tantalum butoxide to produce tantalum-doped silica with increased refractive index (US6, 546,757). The production of silica soot doped with metal oxides including rare earth oxides by spray combustion of organometallic compounds dissolved in siloxane is also described in US 6,739,156. Proposed dopant compounds include alkoxides and β-diketonates. Although mutual solubility can be improved by the addition of a solvent (eg, ethylene glycol monomethyl ether), this patent creates challenges due to supply line plugging caused by easy hydrolysis of the proposed organometallic dopant precursor. Mention getting. Although possible solutions to these challenges have been provided. The use of expensive organometallic dopant precursors means that this method is insufficient as an economically viable method for producing the necessary doped glass on a large scale.

  Furthermore, there are numerous uses for doped vitreous silica, where no suitable organometallic precursor exists, or the high cost of using such compounds would be unacceptable. These include semiconductor elements that are resistant to plasma etching, optical filters, special lamp covers, laser activated glass, and the like. Some of these applications have served in the past by melting natural quartz powder intimately mixed with appropriately refined oxide powders, but for some applications it has been conventionally used today. The method is known to be inappropriate. If it is required to have the highest possible purity without any internal defects, microbubbles, undissolved particles, etc., the doped silica glass is preferably produced from a high purity synthetic raw material, and the glass is highly Preferably, it is uniformly doped, i.e. uniformly doped at the atomic level. In the past, it has been found that for many potential metal dopants it is impossible to achieve on a large scale at an acceptable cost.

  Uniform doping of silica with transition metal oxides, and especially rare earth metal oxides, where special difficulties arise and are serious problems when doping with some more refractory oxides is required Was found with respect to. In addition, the difficulty in obtaining an ultimate solution of rare earth metal oxides in vitreous silica (due to the relative insolubility or “clustering” of metal ions in the glass) helps the solubility and dispersion of rare earth metal oxides. Therefore, it is common to dope with aluminum oxide.

  As mentioned above, potentially volatile rare earth metal precursors are not available or are very expensive and difficult to handle. Thus, it would be more cost effective and more convenient if it was possible to use a relatively inexpensive salt of a suitable metal, or a mixture of metals.

  In US2008 / 0039310, one precursor (silicon tetrachloride) can be supplied as a vapor and a second precursor can be supplied as a spray to the flame. In this case, an atomized spray of an aqueous ethanol solution of lanthanum and aluminum chloride was supplied with silicon tetrachloride vapor, supplied to the flame of an oxyhydrogen burner, and deposited as porous glass on the substrate. It was done. This has been proposed as a potential alternative to soot body solution-doping for the production of such doped glasses. However, it will be difficult to ensure that the lanthanum oxide species are generated in the form of ultrafine particles. If complete evaporation of all precursor species has not occurred before conversion to oxide, oxide formation begins in the condensed phase (in this case, dehydrated droplets of the solution) and the dopant oxide is atomized in the glass. It will not be generated in the form of nanoparticles, where a level of homogeneity is required. And the dopant oxide will not be generated in the form of nanoparticles, where atomic homogeneity is required in the glass. Furthermore, since silica particles and dopant oxide particles and dopant oxides are produced in two different regions of the flame, this method provides a soot body in which the dopant concentration varies with the soot layer and is the most important application. Does not give the ultimate uniformity and / or transparency required in Optical transmittance data was provided in the above patent application for doped glass prepared by solution doping of silica soot porous material, but prepared by spraying a dopant seed solution onto a separated silica synthetic flame. No numerical values were shown for the materials made.

  The potential benefit of providing one or more dopants as an aqueous solution is shown in US 6,705,127, which is one or more soluble dopants (eg organometallics) in non-aqueous silica precursors (eg siloxanes). Compound) may optionally be included with an additional solvent as an aerosol solution in one burner, together with a second spray of aqueous solution dopant in a second burner. In this arrangement, the dopants, such as those present in the silica soot and the second spray, are placed separately in the continuous layer, but cannot lead to the desired dense component mixture in atomic size. In another embodiment, the non-aqueous and aqueous liquids are fed into a single burner and then sprayed into a single flame, but with respect to the burner design that can be achieved or of a quality that the method can tolerate. There is no indication whether it is effective to give.

  US2006 / 0001952 mentions the need for ultimate homogeneity of the glass even when the dopant is refractory, insoluble dopant particles and is prone to form localized enrichment (ie clustering) of the dopant oxide. . For this reason, in the course of the combustion synthesis process, it is desirable that both silica and dopant precursor species evaporate sufficiently and then condense together as nanoparticles of mixed oxide particles. This should lead to close mixing at the atomic level. It has been proposed that this can be achieved using a coaxial oxyhydrogen burner, in which the central nozzle is fed with a mixture of aluminum, sodium and erbium chloride aqueous methanol solutions, and this spray is a high-speed co-axial hydrogen. Atomized by the axial flow, it was itself surrounded by a carbon tetrachloride vapor flow and finally an oxygen flow. The gas velocity is said to be in the range of 0.3 to 1.5 times the speed of sound, and high flame turbulence is said to give intimate mixing of the components. However, as reported in other patents, such high speeds and turbulences can efficiently produce soot in the form of a uniform porous body that is suitable for firing into defect-free and defect-free glass. It does not conduct to collect. Thus, the method may be suitable for the deposition of small amounts of doped products, can be converted to high value optical fiber preforms, and more useful materials can be selected, but relatively large amounts of defects It does not seem appropriate for the efficient production of glass without glass.

  Although the use of salts of dopant species supplied as an atomized aqueous solution to the flame has been proposed in the past, the method is particularly useful when they are relatively non-volatile. It is expected that droplets of such a solution will evaporate to dryness and then decompose into solid or liquid particles, which is unlikely to lead to evaporation, but due to the very high boiling point of the dopant oxide of particular interest In addition, at least some of the resulting oxide particles remain large in size and constitute unacceptable defects in the product glass.

  What is needed is a method of synthesizing particles of dopant oxide, or oxide, in a flame, where all precursors are fed into the reaction zone as a substantially uniform mixture, while each dopant oxidation Ensure that all particles of the product co-condense from the vapor species in the presence of condensed silica particles, otherwise dispersed nanoparticles of dopant oxide of a size comparable to that achieved by condensation from the vapor, That is, it is generated as a submicron size.

  The inventor is able to produce both silica and dopant oxide particles of sub-micron size, i.e., the size of a nanoparticle equivalent to the size of an oxide particle synthesized from a conventional synthetic flame Invented a new way to: Furthermore, the method has been shown to be suitable for the production of both pure and doped synthetic silica glasses, indicating that even homogeneous blends of refractory and relatively insoluble oxides are possible. As a result, uniform doping can now be achieved on an atomic scale.

Graph showing the droplet size distribution of an emulsion of water and cerium nitrate solution in D4N mixed with siloxane with two alternative emulsifiers. 1 is a cross-sectional view of one embodiment of a synthetic burner suitable for use in the method according to the present invention.

  This process is a new variant of the well-known spray pyrolysis method. It is particularly advantageous when used with a chlorine-free silica precursor having a large heat of combustion. Thus, possible silica precursors include siloxanes, the most important of which are hexamethyldisiloxane (HMDS), octamethylcyclotetrasiloxane (also known as OMCTS, D4), decamethylcyclopentasiloxane (DMCPS or D5), and dodecamethylcyclohexasiloxane (DMCHS or D6). Other chlorine-free organosilicon compounds can also be considered for this type of process if they do not undergo unacceptable hydrolysis under the conditions of use, but the precursors of particular interest are siloxanes, especially cyclosiloxanes, and more In particular, D4, D5, and D6, and mixtures thereof. However, other cyclic and chain siloxanes can be used, including homologues, substitutions and mixtures thereof.

  Aqueous or alcoholic solutions of metal salts are usually not miscible with the siloxane, and as noted above, mixing of two separate components (one or both of which can be a spray) is now at ideal homogeneity. Was not achieved until. However, it has been found that if the dopant is supplied as a solution, for example an aqueous solution, it is possible to create the required ultimately homogeneous dispersion of silica and dopant precursor, Supplied as an emulsion in siloxane, ie a water-in-oil emulsion. This emulsion, where all the components of the ultimate glass may be present in a dense and homogeneous mixture, is necessary to produce the ultimate product glass by flame decomposition, hydrolysis, or oxidation. , Sprayed into a synthetic flame, with each droplet in an aerosol containing all components. Uniformly doped silica particle fumes can be produced in this way, collected as soot and then (or simultaneously) fired into glass.

  Synthetic flames are usually supplied by a suitably designed combustion burner, where precursor oxidation is promoted by oxygen or an alternative oxidizing gas, and the additional energy is supplied by a suitable combustible fuel gas. For example, including hydrogen, methane, propane, butane, ethylene, acetylene, and the like. However, in certain embodiments, it may be advantageous to provide such additional energy by alternative means such as plasma heating. Plasma heating can be provided by known plasma heating devices, including induction plasma torches, arc plasma torches and microwave plasma devices. Similarly, general combustion can be supported by such additional electrical heating means.

  Accordingly, the present invention, in one aspect, provides a method for making a silica-based glass comprising the following steps: forming an emulsion of an aqueous phase in a non-aqueous liquid silica precursor material; As a synthetic flame, so that the precursor material is converted into a silica-containing soot in the flame; and subsequently as a porous soot for sintering into glass, or directly as a substantially fine Collect soot on the substrate as glass without holes.

  In certain embodiments, the silica-based glass is doped or undoped (ie, substantially pure) synthetic vitreous silica. The aqueous phase constitutes the discontinuous phase of the emulsion and can be comprised of pure (or substantially pure) water or a suitable aqueous solution, as detailed below. In many embodiments, each droplet in the spray contains all the components required to convert to silica-based glass by oxidation or hydrolysis in a flame. For example, each droplet contains an aqueous phase emulsion in a non-aqueous liquid silica precursor material, as well as some (or all) dopant.

  Emulsification can be facilitated by the addition of one or more emulsifiers. A wide range of surfactants exist and are considered as emulsifiers for this application. It may be advantageous to use two or more surfactants together, where it may be advantageous to have different HLB (hydrophilic lipophilic ratio) values. Thus, surfactants with HLB values of 3-9 may be suitable, but this range is not limiting. Examples of suitable emulsifiers are nonionic surfactants free of metal impurities, including silicone surfactants such as alkoxylated derivatives of silicone polymers, polymethylsiloxanes (which may be alkyl substituted) and polyethers For example, a copolymer with polyethylene glycol (PEG) and / or polypropylene glycol (PPG). Suitable emulsifiers are Tegopren® 7008, polyether alkylpolymethylsiloxane (Evonik Degussa GmbH, Rodenbacher Chaussee 4, 63457 Hanau, Germany) and BC99 / 012 (Basildon Chemical Company Limited, Kimber Road, Abingdon, OX14 1RZ, OX14 1RZ, UK), but many others are available from various manufacturers.

  The aqueous phase in the emulsion may comprise one or more aqueous metal salt solutions, for example a mixture of rare earth metals and aluminum salts. Emulsification to form controlled sized aqueous solution microdroplets in oil media is achieved by standard methods (eg, high speed blender, high pressure homogenizer, colloid mill, high shear disperser, sonication membrane homogenizer, etc.) And can be facilitated by adjusting the temperature, etc. by the addition of a suitable sea surface active agent. If the emulsion is sufficiently stable, the homogenization process can be performed in a separate process prior to feeding to the equipment. Alternatively, the emulsification can be arranged in a straight line in a vessel upstream of the burner or, for example, a burner equipped with an ultrasonic horn adapted to supply and atomize two liquids (eg Sono-Tek Corporation , Milton, NY 12547, available from the United States).

  By suitable methods, the aqueous microdroplets in the siloxane emulsion (for example) are sized such that each droplet of the atomized spray consists of a siloxane droplet containing an aqueous microdrop such as a microemulsion. You can be sure. Thus, when the emulsion is atomized into oil droplets of 20-100 μm size, they each contain a large number of significantly smaller aqueous droplets. Typically, the majority of aqueous microdroplets have a diameter of less than 2 μm. Upon heating in the flame, the water in the aqueous microdroplets can be superheated and then explosively evaporate, and thus the refractory dopant oxidation formed upon evaporation of the water and subsequent decomposition of the metal salt This is thought to help reduce the particle size of the object. At the high temperatures present in the flame, extensive evaporation of the dopant oxide species occurs, and thus the required doped silica will actually be achieved by co-condensation of the constituent oxide species. However, even if the refractory dopant species is not sufficiently evaporated, the precursor solution microdroplets are split and evaporated to dryness, and then fully oxidized in flight, sub-oxide or mixed oxide. Produces micron particles. These particles are of a size comparable to that achieved by nucleation and growth from steam and are intimately mixed with silica nanoparticles generated in the same region of the flame.

  The collapse of water droplets gives the advantage that siloxane droplets are also broken, ensuring better mixing of siloxane and oxygen, more uniform combustion and a more compact combustion zone, and higher potential capture efficiency. Lead. This overcomes the aforementioned disadvantages of atomization burners. That is, spraying leads to oversized droplets that can lead to the arrival of partial oxidation, oversized particles on the substrate, and their incorporation in the porous body, wart-like growth of the soot body and in the resulting glass Overcoming the disadvantages (as described in US 6,260,385) resulting in potential microbubbles.

  The new method can be advantageous for the production of both pure and doped synthetic silica glasses when the dopant oxide is highly refractory. Thus, when used to produce doped silica glass, ensure that silica particles that ultimately condense from the vapor are actually nucleated and grow on the refractory dopant oxide nanoparticles. . The result is that the doped soot that is deposited and then dried, unlike the previously proposed method, produces a homogeneous dopant solution in silica glass at a tunable and predictable concentration. .

  In this way, it is possible to blend a wide variety of dopant species into the product glass. Such dopant species can be introduced into the aqueous phase of the water-in-oil emulsion as a water-soluble salt of the compound. Typically these include solutions of metal nitrates, chlorides, acetates, etc., or non-metallic elemental compounds such as boron, phosphorus, etc.

  A new method of combustion of aqueous emulsions in siloxanes was originally developed as a route for the production of doped glass, but pure water emulsions in non-aqueous liquid silicon precursors as atomized sprays from combustion in flames Has also been found to provide an improved method for producing pure synthetic silica glass.

  It has been found that the addition of ionic species to the aqueous solution improves the stability of the emulsion and allows the achievement of relatively small droplets in the emulsion. This can be potentially beneficial not only in the production of doped silica glass as described above, but also in the production of undoped glass. Thus, it may be advantageous to add the appropriate ionic species to the pure water emulsion, which is a species that decomposes or oxidizes in the flame without causing significant impurities in the silica soot to be deposited. It is. Potential additives include mineral acids such as hydrochloric acid, nitric acid and sulfuric acid; carboxylic acids such as formic acid, acetic acid, propionic acid and the like; or organic bases such as methylamine, trimethylamine and the like, and suitable counterions such as Hydrochlorides; and degradable salts such as ammonium chloride, nitrates, sulfates, formates, acetates and the like. The use of ammonium nitrate or other high bound oxygen content salts is of particular interest. Such species are advantageous in generating significant gases upon pyrolysis and in favoring the degradation of precursor particles in the aerosol and sufficient oxidation to ultrafine silica particles. While leaving substantially no different species of residue on the silica.

  The use of a water-in-oil emulsion, in which the aqueous phase contains one or more dopant species precursors in the form of dissolved salts, but a large amount of doped synthetic silica glass at a predictable concentration. Provides a cost-effective method in which the dopant species are ultimately dispersed in atomic size and are free of particle inclusions. It is expensive and difficult to handle organometallic dopant precursors; if the dopant oxide particles are not sufficiently dissolved, the risk of microbubbles and inclusions in the glass; and the uniform concentration of the dopant in the glass Or the difficulty of predicting the average level of dopant in the final glass based on the deposition conditions.

  The method of the present invention is particularly useful for the production of silica glasses doped with rare earth metal oxides and other refractory oxide species, but is not limited to these species. Indeed, this method can be used to produce glass products doped with a wide range of metal oxides selected from the Periodic Table (IUPAC version) Groups 1-16 and the lanthanide and actinide series. It can also be used to formulate certain non-metallic dopants, including, for example, boron and / or phosphorus oxides.

  As required in some applications, if it is desired to have a gradient in the dopant level in the glass, this can be achieved by gradual variation in the concentration of the particular dopant species present in the emulsion. In some embodiments, it may be appropriate to provide one or more dopant species dissolved in a non-aqueous phase, but one of the main advantages of the present complement is that such as siloxane It is to provide a cost-effective method of producing homogeneously doped glass using an aqueous emulsion comprising a non-aqueous, chlorine-free silica precursor and a dissolved dopant precursor in the form of an inexpensive salt. .

  As mentioned above, the method of the present invention can produce doped glasses, where the dopant species are desired to be achieved at a much greater level of homogeneity than that achieved with the originally produced glass. With respect to the atomic level, it is substantially homogeneously distributed. For example, it may be desirable to achieve a more uniform dopant level than originally achieved, or it may be necessary to take steps to improve the uniformity of refractive index, thermal expansion coefficient or other properties, And, for example, a more uniform OH level can be required than can be obtained with initially produced glass. Under these circumstances, the overall homogeneity of the glass can be improved by applying a subsequent homogenization process followed by a suitable annealing operation. Some homogenization can be achieved by a glass reflow process that gives an alternative shape, especially with a relatively large cross-sectional area. The homogenization process described in US 5,443,607 and US 5,790,315, where the glass bodies are mixed by twisting, is particularly suitable for the production of optical glass with the highest homogeneity.

  Supplying the dopant species to the flame as a solution in the aqueous phase of the emulsion is considered one of the most important applications of the present invention, and in certain circumstances, suspended in the aqueous phase as a dispersion of ultrafine particles or second It is desirable to provide certain dopants as an emulsion of the following, i.e. as an oil-in-water emulsion present as microdroplets suspended in a non-aqueous chlorine-free silica precursor.

  An additional feature of the present invention is that the use of an emulsion of water in the siloxane spray changes the combustion characteristics so that the flame results in reduced levels of nitrogen oxides. This represents an important advantage when used on a large scale in situations where NOx emissions are limited. Accordingly, the present invention provides a method for reducing the NOx emissions of atomized burners used in the production of silica-based glasses, wherein the atomized burner is a non-aqueous chlorine-free liquid silica in the above-described silica production process. An emulsion of the aqueous phase in the precursor is provided.

Examples Experiments were carried out with emulsions of water or aqueous solutions in the respective siloxanes or mixtures. A mixture with the appropriate composition 77% D4, 21% D5, 2% D6 is supplied as “D4N” (Momentive Performance Materials, 22 Corporate Woods Boulevard, Albany, NY 12211, USA). This mixture did not undergo freezing at room temperature and was used for testing, but obviously pure siloxane could alternatively be used in this method.

A typical emulsion is as follows.
Emulsion
A typical emulsion was made as follows. To 920 mL of D4N, 30 mL of emulsifier Tegopren® 7008 was added. Then 50 mL of water is added to the non-aqueous phase and the mixture is used with a high speed homogenizer (Ultra-Turrax, Model No. T125, IKA = Werke GmbH & Co. KG, Janke & Kunkel-Str. 10, 79219 Staufen, Germany). First emulsified. The emulsification was completed by vigorous stirring of the container in the ultrasonic bath. The droplet size distribution of the aqueous phase of the resulting emulsion was measured by a laser diffraction particle analyzer, Model LS230 (Beckman-Coulter UK Inc., High Wycombe, HP11 1JU, UK). This distribution is shown in FIG. The diameter of the aqueous phase microdroplets is usually less than 3.5 μm, and the majority of such microdroplets are less than 2 μm in diameter.
Emulsion 2.
An aqueous solution of cerium (III) nitrate (8 g) in deionized water (50 mL) was added to 920 mL of D4N containing 30 mL of Tegopren® 7008. The mixture was emulsified as described above and the droplet size distribution was measured as described above, yielding the results shown in FIG. The diameter of most of the aqueous microdroplets is clearly less than 1 μm.
Emulsion 3.
An aqueous solution of cerium (III) nitrate (40 g) in deionized water (50 mL) was added to 920 mL D4N containing 30 mL BC99 / 012. The mixture was emulsified as described above and the droplet size distribution was measured as described above, yielding the results shown in FIG. Despite the high concentration of dissolved salt, all aqueous droplets are smaller than 1.5 μm in diameter.

  Emulsions such as these can be supplied to the atomization burner, a simple example of which is schematically shown in FIG. The burner assembly may consist of three elements: an atomizing nozzle, a pilot burner and an auxiliary burner.

  The spray is supplied by simple stainless steel atomization 10 (Model No. AL / 06, Delavan Ltd., Widnes, WA80Rj, UK), which is a siloxane mixed liquid (ie, selected water-in-oil emulsion) 11 Along with the flow, it can be supplied by a pulseless metering pump (not shown) and atomized with a flow of atomized gas 12, such as oxygen. This produces a fine droplet spray 13. Typical average droplets for spray from such nozzles under the conditions of use are in the range of 20-100 μm. Thus, when operated with a water-in-oil emulsion, each droplet in the spray contains numerous microdrops of the aqueous phase and appears to incorporate all the components of the final pure or doped silica soot. , Requiring a reaction with oxygen to produce soot particles of substantially uniform composition.

  The nozzle 10 may be placed under a stainless steel pilot burner 14, incorporates a ring 16 of a 1 mm diameter orifice that appears on a circle with a radius of 11 mm and is tilted in the direction of the burner's main axis. Supplied with hydrogen 15, this comprises a row of pilot flames 16 to stabilize the combustion of the siloxane spray. Air flow 17 is caused by the atomizer spray and helps keep the atomizer nozzle surface free of silica deposits. As an alternative to this burner, the nozzle 10 and pilot burner 14 can be hermetically coupled and the arrangement can be adapted to provide a controlled flow of selected gas in an annular gap, indicated at 17.

  The total operating capacity of the burner can be adjusted accordingly with the aid of an auxiliary burner 18, which can be made of fused silica in the form of a coaxial burner, supplied with oxygen through port 19 and hydrogen through port 20.

  Such burners can be used to convert aqueous emulsions in siloxanes into pure and doped silica soot and then calcined into pure and doped silica glass. By adjusting the liquid and gas flows and the size of the annular gap 17 and the distance separating the burner and the substrate (not shown), it is possible to optimize the deposition efficiency for a given soot density. is there. Under some circumstances, deposition efficiencies have been found to be limited, with higher deposition efficiencies leading to nodular growth of soot bodies and producing glasses with unacceptable defects upon firing. This effect is thought to be due to the incorporation of excessive and insufficiently oxidized particles into the soot body rather than ultrafine oxide particles. When nodule formation occurred, high-speed photographs around the flame during operation revealed evidence that could be considered to correspond to such oversized particles. These can cause inclusions or microbubble regions in the glass as it is fired, and such defects are unacceptable in the product.

In a comparative example, it is possible to show deposition conditions that lead to nodule growth. A burner is fed with the above mixed siloxane at a rate of, for example, 0.72 L / hour, atomized in oxygen (0.70 sm ³ / h (ie standard cubic meter / hour)) and piloted at 0.73 sm ³ / h With hydrogen flow to the jet and oxygen flow to the auxiliary burner at 2.10 sm ³ / h and hydrogen flow at 1.75 sm ³ / h. When the burner is fed with a similar stream of the emulsion under otherwise similar conditions, no-bar acceptable soot deposition can be achieved, and the resulting pure silica soot body is vacuumed at a temperature of 1500 ° C. for 1 hour. By heating under, it can be fired into glass free of bubbles and inclusions. If necessary, it can be subjected to a dehydration and purification process by heating (for example) in a chlorine-free atmosphere prior to calcination. Firing in hydrogen or helium is also possible.

  The addition of a suitable emulsifier is beneficial in helping to achieve a stable water-in-oil emulsion, where the aqueous droplets are small in size and this size distribution can be maintained over time. However, in some situations it is possible to operate with emulsions without added emulsifiers. Such an emulsion does not appear to be relatively stable, but can still be used in the process of the present invention. For example, when separately supplying a siloxane without added emulsifier and an aqueous phase to a burner incorporating an ultrasonic atomizing horn, the two components are melted together under the influence of ultrasonic vibrations at the surface of the horn. An aqueous microdrop emulsion is thereby formed within each siloxane droplet that is fed into a flame and reacts to form a suitable pure or doped silica soot. Thus, the presence of added emulsifier is not an essential feature.

  Achieving acceptable soot deposition using an emulsion of an aqueous solution of a dopant salt in a siloxane precursor, such as emulsion 2 or 3 above, and thus homogeneously doped at the atomic level, It is possible to achieve a uniformly doped soot body that can be fired into glass without bubbles and inclusions.

  The above experiments demonstrate the feasibility of the method of the present invention and further combustion of a water emulsion or an emulsion of an aqueous solution of one or more metal salts in a non-aqueous (eg, siloxane) precursor; By collecting the product as a porous material suitable for subsequent firing into glass, it is shown that pure and doped synthetic silica glass can be achieved. With a suitable burner and by raising the temperature of the substrate, it would be possible to fire the soot particles directly into the doped silica glass without pores.

  These experiments can be done with the simplest burner, but a more sophisticated burner can be used to generate an emulsion spray and supply it to a synthetic flame, and various methods of atomization are possible. It is clear that there is. For example, the emulsion may be atomized by pneumatic means (eg, atomized gas stream) or ultrasonic means (eg, use of an ultrasonic horn). In certain embodiments, it is possible to achieve in-situ emulsification of the liquid supply, eg in an ultrasonic horn (which may be placed in a burner) adapted to handle two liquid supplies. By supplying liquid. Alternative burners may include coaxial burners that use hydrogen, natural gas, propane, etc. as the fuel, or may use a premixed gas supplied as a jet stream to the flame, for example. An inert gas such as nitrogen or argon is used as an alternative atomization gas or as a gas that separates from the reaction gas to help cool the burner element or to avoid surface build-up of burners etc. And, if necessary, can also be used as a diluent gas in a reactive gas stream. It is clear that oxygen is the most convenient oxidizing gas, but other gases (eg air, ozone, nitrogen oxides etc.) are possible in principle.

  When using a single non-aqueous precursor (eg siloxane) or a precursor mixture such as an aerosol spray supplied to a silica synthesis burner, pure water or an aqueous of an ionic species suitable as an emulsion in the non-aqueous precursor Adding a solution emulsion can improve the combustion efficiency of the precursor, and can further improve the potential collection efficiency and quality of the resulting pure silica glass product. Furthermore, by replacing pure water with an aqueous solution of one or more metal compounds, or a water colloidal suspension of such compounds, it is possible to deposit homogeneously doped silica soot, thus eliminating bubbles and inclusions. No homogeneously doped glass can be obtained.

Claims (27)

  Forming an emulsion of the aqueous phase in a non-aqueous liquid silica precursor material; feeding the emulsion as a droplet spray into a synthetic flame, thereby converting the precursor material into a silica-containing soot in the flame And then collecting the soot on the substrate as a porous soot body to be sintered into the glass or directly as a substantially pore-free glass, and a method for producing a silica-based glass comprising the steps of: .   The method according to claim 1, wherein the synthetic flame is supplied by a synthetic burner.   The manufacturing method according to claim 1, wherein the synthetic flame is supplied or increased by a plasma heating device.   The method according to any one of claims 1 to 3, wherein the silica precursor material is a non-aqueous silicon compound containing no chlorine.   The production method according to claim 4, wherein the non-aqueous, chlorine-free silicon compound is a siloxane compound or a mixture of the compounds.   6. The method of claim 5, wherein the siloxane compound is hexamethyldisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, or a mixture of two or more thereof.   The production method according to any one of claims 1 to 6, wherein the aqueous phase of the emulsion further contains an emulsifier.   The production method according to claim 7, wherein the emulsifier is a silicone surfactant.   The production method according to claim 1, wherein the aqueous phase is pure water and the silica-based glass is pure silica.   9. The aqueous phase is a solution of water and ionic species, the ionic species helping to achieve a droplet emulsion and / or ensure improved combustion of the spray. The manufacturing method as described in.   The production method according to claim 10, wherein the ionic species is ammonium nitrate.   9. A process according to any one of the preceding claims, wherein the aqueous phase comprises a precursor solution of one or more dopant species.   13. The method of claim 12, wherein the aqueous phase comprises a precursor solution of one or more metal dopant species.   The metal dopant species comprises one or more compounds that can be converted into one or more metal oxides selected from the Periodic Tables 1-16, and lanthanides and actinides in a synthetic flame. 14. The production method according to 13.   15. A method according to any one of claims 1 to 8 or 12 to 14, wherein one or more non-metallic dopant species are incorporated into the glass.   The method of claim 15, wherein the non-metallic dopant species comprises an oxide of boron or phosphorus.   The manufacturing method of any one of Claims 1-16 in which the droplet in a spray contains all the components requested | required of silica type glass about the conversion by the oxidation or hydrolysis in a flame.   18. A process according to claim 17, wherein the droplets in the spray comprise an emulsion of an aqueous phase in a non-aqueous liquid silica precursor material.   The process according to claim 18, wherein the diameter of the majority of the droplets of the aqueous phase in the emulsion is smaller than 2 m.   17. A process according to any one of claims 18, 19 and 12-16, wherein each droplet in the spray comprises an emulsion of an aqueous phase in a non-aqueous liquid silica precursor material further comprising a dopant species.   21. A process according to any one of claims 1 to 20, wherein the emulsion is atomized in a synthetic burner by means of aerodynamic means.   The method according to claim 21, wherein the aerodynamic means comprises a atomized gas stream.   21. A process according to any one of claims 1 to 20, wherein the emulsion is atomized in a synthetic burner by ultrasonic means.   The manufacturing method according to claim 23, wherein the ultrasonic means includes an ultrasonic horn.   The process according to any one of claims 1 to 24, wherein the emulsification occurs in or near the burner and is facilitated by an ultrasonic horn adapted for simultaneous feeding and emulsification of the two liquid phases.   26. A method according to any one of claims 1 to 25, wherein the initial glass product is subjected to a subsequent homogenization process.   A method for reducing NOx emissions from atomized burners used in the manufacture of silica-based glass, wherein the atomized burner is provided with an emulsion of an aqueous phase in a non-aqueous, chlorine-free liquid silica precursor The manufacturing method of any one of Claims 1-26.

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